A digital printer generally requires a pixel processing unit to perform various operations on the raster data, or original image data, prior to printing an image. These processes may include scaling, linearization and halftoning, amongst others.
The process of halftoning, for example, is used to generate a halftone image, which consists of a selectively positioned arrangement of dots of fixed tone levels. This arrangement creates the illusion of a continuous tone, or “contone”, image. Thus, halftoning is a process that allows a continuous tone image to be modified in order that it may be represented by a device that can only represent a finite number of tone levels.
Various halftoning methods have been devised. Initially, “dithering” or matrix-based techniques were used to halftone images. In such techniques, a two-dimensional array of values is mapped over a continuous tone image. The data in each pixel in the continuous tone image is compared to the matrix value for that pixel. If the continuous tone value is greater than the matrix value, then a dot is printed in the corresponding position in the halftone image. More recent refinements to the halftoning process include “error diffusion” type halftoning processes. Error diffusion halftoning is a neighbourhood process. That is to say, when determining whether or not to print a pixel in a halftone image, the value of the corresponding pixel in the continuous tone image is considered together with information pertaining to pixels neighbouring that pixel. Such processes often give rise to halftone images with reduced grain compared to images produced by dithering or matrix-based techniques.
Modem inkjet devices and printing methods usually employ considerable processing power to carry out such pixel processing. Nevertheless, the time required to process print data prior to printing an image is often appreciable. In turn, this required processing time may adversely affect the throughput of the printer. At the same time, the importance placed on the throughput of inkjet printers has risen dramatically over recent years. Consequently, manufacturers have embarked on a process of continually improving their inkjet devices to give improved throughput in order to secure a competitive edge in the marketplace.
It would therefore be desirable to provide method and apparatus which allows at least some images to be processed prior to printing in a manner which allows the throughput of the printer to be increased.